The challenge
Completed in 1871, the fort takes the form of a semi-circular arc that was designed to protect the deep-water anchorage at Spithead. Prior to the First World War, the walls were strengthened with earthwork embankments, and its military use continued beyond 1945 with stints as a signal station and storage units. After falling into serious disrepair prior to its recent sale for £1.4 million, the structure is currently undergoing extensive enablement works, with permission granted to convert the fort into an exclusive enclave of 26 homes.
Speaking about the project, our Senior Monitoring Engineer, Allan Martin, explained: “The planning permission for the project allows the developers to remove much of the old earthworks and the seaward wall to the old fort, along with the hundreds of tonnes of mass concrete which had been poured on top of the original masonry structure to strengthen it during the Second World War.
“We were approached by Hampshire-based BD Nuclear, a demolition specialist, and asked to provide a monitoring package for the series of arches that support the fort’s roof. This was imperative to ensure there was no damage caused by the hydraulic breaker being used to remove the concrete."
The solution
Allan continued: "The bespoke monitoring package included 22 sensors to monitor site-wide vibration, plus a further five wireless crack sensors to check if any existing cracks were getting wider.
“Due to site security concerns, we were also tasked with providing a full wireless connection, wireless logger, and wall-mounted battery box, meaning that everything could be installed and secured from public access.”
Unusually, due to the scale of the building, we customised the system to feature ‘click-fit’ sensors, meaning that the equipment could be rapidly rearranged to monitor three adjacent arches at a time as work progressed. BD Nuclear’s own personnel were also trained in how to correctly demount and relocate sensors as required, offering significant cost savings for the client.
Jack Dougan, Operations Director for BD Nuclear, commented: “The handiest part of having this training was that there was no down time for us, and we were fully in control of the monitoring. When we were ready to move the hydraulic breaker, we could easily relocate the sensors to the arches below and then carry on the following day with no problems at all.
“Most importantly, there were no alerts triggered due to movement and Heritage England were fully satisfied with the results. I would always use Mabey Hire for monitoring requirements in the future, as well as for much of our temporary works hire needs.”
One of the benefits offered to customers by our structural monitoring service is 24/7 access to real-time data via our Insite system. Readouts from the various sensors are relayed to the main frame computer and, if pre-set limits are exceeded, will trigger alerts and enable work to be halted.
Interestingly on the Fort Gilkicker project, the monitoring sensors showed a steady rate of recovery in the structure as the overburden was taken away, as well as tracking thermal movement as the earthworks were removed and the sun began to reach the historic masonry.
